Heering, Walther, 1902. Das Rolleiflex-Buch : Lehrbuch fur Rolleiflex und Rolleicord. Harzburg, 1936. If you read German this is an excellent handbook that not only covers the Rollei cameras and accessories on the market at that time but also describes techniques for composing pictures and using filtres.

Heering, Walther. The Golden Book of the Rolleiflex. Harzburg: Dr. W. Heering, 1936. “Published in German, English, and French. In order to facilitate reference, the illustrations are numbered uniformly in all versions, viz. from 53 to 180. This volume contains text pages 1-20 and plates 53-180”–P. 20. “Winners in the European 1935 rolleiflex-rolleicord-competition”: p. 14-19.